In many electric applications, there is a need of measuring current that flows through a primary conductor. A very useful device for measuring such a current, especially an alternating current, in a contactless manner is a Rogowski type coil. A Rogowski coil is a coil having multiple windings around an elongated body. The elongated body is at least partially (i.e. by at least half a turn) wrapped around an axis of the primary conductor. Further, both terminals of the coil are at the same proximal end of the coil. This is achieved by a lead connecting a distal end of the coil to the corresponding terminal, the lead extending along the elongated body and through a middle of the coil. When the Rogowski coil is (fully or partially) wrapped around the axis of the primary conductor, a voltage induced between the terminals is proportional to the derivative of the current flowing through the primary conductor.
For measuring the primary conductor current with high accuracy, high-end Rogowski coils in compliance with IEC 60044-8 Class 0.2 are available. If these high-end Rogowski coils have a high winding homogeneity and are arranged precisely in relation to the primary conductor, and if high-end readout electronics are used, accurate measurements over a long period of time are possible.
However, these high-end Rogowski coils and readout electronics are costly to realise and to arrange precisely. Therefore, there is a need for a lower cost solution using more cost-effective components. More cost-effective Rogowski coils, such as flexible Rogowski coils, however, typically have poorer winding homogeneity. For this and other reasons, there has been a problem of having to calibrate these Rogowski coil periodically due to declining measurement accuracy. The calibration is necessary for diagnosing the installation and for verifying that the current measurement device is functioning and that its accuracy is sufficient.
For the purpose of calibration, the Rogowski coil is typically removed from the primary conductor and applied to test equipment generating a calibrated primary current. A similar procedure has to be applied to the readout electronics, which has to be calibrated as well. Hence, the calibration procedure generally requires the primary conductor or at least the current measurement device to be disconnected or shut down. Furthermore, these calibration procedures have only limited capability to account for drifts in measured current due to changes in the environment of the measurement apparatus, such as temperature fluctuations or fluctuations in other operating conditions.
In EP Patent Application EP 12002767, a system for performing background calibration to both a Rogowski coil and to readout electronics has been proposed. The solution involves a third conductor winding along the Rogowski coil, which is adapted to receive a calibration current signal. However, there is a need for an even more accurate calibration system that takes into account additional sources of error.
US 2004140879 discloses a non-Rogowski type current sensor based on a transformer with closed probe core and a closed compensation core, both made of magnetic material. The setup increases the dynamic measurement range of the sensor by feeding a current into the compensation winding of the setup which generates a compensating magnetic flux.